Q: Why are priests adding the Memorare and/or the Prayer to St. Michael to the Mass? If it is added, when should it be recited?
A: The Memorare and Prayer to St. Michael are not part of the mass, so a priest who adds them does so on his own initiative. You may ask the individual priest why he does it. In my view, the mass does not need any fixing. If the community discerns it needs stronger Marian devotion or protection under the patronage of St. Michael, then such intentions are best crafted within the Universal Prayer. That is exactly what that part of the mass is for.